Transformer Based Gate Drive Circuit

ABSTRACT

A gate drive circuit for generating asymmetric drive voltages comprises a gate drive transformer comprising: a primary winding responsive to a pulse width module (PWM) input signal to generate a bipolar signal having a positive bias voltage and a negative bias voltage; and a secondary winding responsive to the bipolar signal to generate a PWM output signal. A first charge pump is connected to the secondary winding responsive to the PWM output signal to generate a level shifted PWM output signal. A second charge pump is connected to the secondary winding to generate a readjusted PWM output signal by decreasing at least a portion of the level shifted PWM output signal. A gate switching device is connected to the first charge pump and second charge pump. A level shifted PWM output signal establishes an ON condition and the readjusted PWM output signal establishes an OFF condition of the gate MOSFET.

RELATED APPLICATIONS

This is a divisional application of U.S. application Ser. No.15/413,166, filed Jan. 23, 2017, entitled “Transformer Based Gate DriveCircuit,” which is incorporated by reference in its entirety herein.

GOVERNMENT LICENSE RIGHTS

This invention was made with government support under Contract No.N00019-13-C-0128 awarded by the Department of Defense. The governmenthas certain rights in the invention.

BACKGROUND

Modern power electronics often make use of metal oxide semiconductorfield effect transistors (MOSFETs) and insulated gate bipolartransistors (IGBTs) in many applications. Power converters are comprisedof a power circuit or topology consisting of power switching devicessuch as MOSFETs and IGBTs, control circuits that regulate the powerconversion operation and gate drive circuits that serve as an interfacebetween the two. Gate drive circuits are required to switch the MOSFETsand IGBT devices ON and OFF to control and condition the powerconversion function. Gate drive circuits serve as the interface betweenthe control circuitry and the power circuitry by conditioning andconverting the Pulse Width Modulation (PWM) control signal to regulatethe power conversion operation as required by the characteristics of thepower switching device used in the power circuit.

All switching power converter topologies require one or more gate drivecircuits depending on the number, type and electrical connection of thepower switching devices used therein. MOSFET and IGBT type devices arecontrolled by applying a voltage between a control terminal,traditionally referred to as the “gate” and a reference terminal,traditionally referred to as the “source” or “emitter” respectively. Apositive voltage at the gate with respect to source or emitter, wouldswitch an N-channel MOSFET or IGBT ON, whereas a negative or zerovoltage at the gate with respect to the source or emitter would switchthe device OFF.

Gate drive circuits with magnetic transformers are commonly used toprovide galvanic isolation between the control circuit and the powercircuit. Transformer isolated circuits provide a robust, high speed, lowloss and low cost implementation of the gate drive circuit for mostswitching devices. Transformers require a balanced volt-time product inthe applied drive signal to prevent saturation. As a result, they aregenerally more readily applicable to power switching devices that cansupport a symmetric, bipolar gate drive voltage to control their ON/OFFbehavior. Silicon based MOSFETs or IGBTs are able to support such asymmetric, bipolar drive voltage.

However, next generation devices such as Silicon Carbide (SiC) MOSFETsdo not support a symmetric gate drive voltage. The SiC MOSFET, forexample, requires, at its gate terminal, 20V to be switched ON and −5Vto be switched OFF. Transformer isolated circuits used in combinationwith DC blocking capacitors can be used with limited success but cannotgenerate controlled voltage levels for turn-ON and turn-OFF independentof operating duty cycle without compromising volt-time product of thetransformer. To overcome this limitation, implementations of gate drivecircuits using auxiliary voltage sources to generate the turn-ON andturn-OFF voltage levels, which are high in component count, cost and lowin efficiency are used.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of the invention will be apparent from thedetailed description which follows, taken in conjunction with theaccompanying drawings, which together illustrate, by way of example,features of the invention; and, wherein:

FIG. 1 is block diagram of a gate drive circuit in accordance with anexample of the present disclosure.

FIG. 2A is an example schematic illustration of a transformer based gatedrive circuit with additional secondary windings in accordance with anexample of the present disclosure.

FIG. 2B is an example illustration of various waveforms for thetransformer based gate drive circuit of FIG. 2A.

FIG. 3A is an example schematic illustration of a transformer based gatedrive circuit with charge pumps in accordance with an example of thepresent disclosure.

FIG. 3B is an example illustration of various waveforms for thetransformer based gate drive circuit of FIG. 3A.

FIG. 4A is an additional example schematic illustration of a transformerbased gate drive circuit with charge pumps in accordance with an exampleof the present disclosure.

FIG. 4B is a schematic illustration of various waveforms for thetransformer based gate drive circuit of FIG. 4A.

Reference will now be made to the exemplary embodiments illustrated, andspecific language will be used herein to describe the same. It willnevertheless be understood that no limitation of the scope of theinvention is thereby intended.

DETAILED DESCRIPTION

As used herein, the term “substantially” refers to the complete ornearly complete extent or degree of an action, characteristic, property,state, structure, item, or result. For example, an object that is“substantially” enclosed would mean that the object is either completelyenclosed or nearly completely enclosed. The exact allowable degree ofdeviation from absolute completeness may in some cases depend on thespecific context. However, generally speaking the nearness of completionwill be so as to have the same overall result as if absolute and totalcompletion were obtained. The use of “substantially” is equallyapplicable when used in a negative connotation to refer to the completeor near complete lack of an action, characteristic, property, state,structure, item, or result.

As used herein, “adjacent” refers to the proximity of two structures orelements. Particularly, elements that are identified as being “adjacent”may be either be abutting or connected. Such elements may also be nearor close to each other without necessarily contacting each other. Theexact degree of proximity may in some cases depend on the specificcontext.

An initial overview of technology embodiments is provided below and thenspecific technology embodiments are described in further detail later.This initial summary is intended to aid readers in understanding thetechnology more quickly but is not intended to identify key features oressential features of the technology nor is it intended to limit thescope of the claimed subject matter.

As mentioned, power converters have been widely used to provideregulated power supplies. In one aspect, power switching devices used inpower converter topologies, may either use a transformer based gatedrive circuit, isolated power supplies, and/or level shifters to createthe necessary drive voltages to turn the power switching devices on andoff. However, gate drive circuits, such as floating gate drive circuits,using isolated power supplies or level shifters are high incomponent-count, limited in their frequency of operation and are not asefficient as a transformer based gate drive circuit.

In one aspect, in order to achieve high-speed speed gate drivecircuitry, switch frequencies (e.g., 200-250 kilohertz) and eliminateisolated bias power supplies, the present technology provides for atransformer based gate drive circuit. A transformer based gate drivecircuit can be compact, robust, and enable high switching frequency.Transformers are inherently symmetrical devices driven by equalvoltage-time product in both the positive and the negative directions.As such, a transformer based gate drive circuit cannot provideasymmetric drive voltages due to the need to maintain balanced volt-timeproduct in the transformer. Hence, a shortcoming of this scheme is thatthe same magnitude of the voltage-time product is needed to drive thegates in the negative direction as well as the positive direction.

Accordingly, the present technology provides for an enhanced transformerbased gate drive circuit to generate asymmetric gate drive voltages toturn-on and turn-off a gate switching device (e.g., an electricalswitch), such as MOSFETs and IGBTs, independent of duty cycle withoutcompromising the voltage-time product of the transformer The transformerbased gate drive circuit can be efficient, compact, and robust and canrequire minimal peripheral circuitry to meet switching requirements ofthe power device used. In one aspect, the transformer based gate drivecircuit can comprise a transformer having a primary winding and one ormore secondary windings to generate the turn-on and turn-off voltages todrive the gate of the switching device. Charge pumps can be coupled tothe one or more tapped secondary windings of the transformer to generateasymmetric gate drive voltages to turn-on and turn-off gate switchingdevices (e.g., MOSFETs and IGBTs). Thus, any need for auxiliary biascircuits for the turn-on and turn-off voltage levels can be eliminated.In an additional aspect, the transformer based gate drive circuit canprovide a primary winding and a single secondary winding and two chargepumps.

The present technology provides various embodiments of transformer basedgate drive circuits for a power switching device. In one aspect, thevarious embodiments of gate drive circuits achieve galvanic isolationusing one or more transformer windings and generate asymmetric turn-onand turn-off voltage levels at the gate of the switching device withoutthe need for external isolated bias voltage sources. The switchingdevice may be a metal oxide semiconductor field effect transistors(MOSFET) and/or an insulated gate bipolar transistors (IGBTs). As usedherein, each of the representative switching devices may be either theMOSFET and/or an insulated gate bipolar transistors (IGBTs) anddiscussions herein pertaining the MOSFET may also apply to IGBT and visaversa.

In one aspect, gate drive circuit for generating asymmetric drivevoltages is provided. The gate drive circuit comprises a gate drivetransformer that may include a primary winding responsive to a pulsewidth modulated (PWM) input signal to generate a bipolar signal having apositive and negative voltage levels; and a secondary winding responsiveto the bipolar signal to generate a PWM output signal. The gate drivecircuit may include a first charge pump electrically connected to thesecondary winding responsive to the PWM output signal to generate alevel-shifted PWM output signal. The gate drive circuit may include asecond charge pump electrically connected to the secondary winding togenerate a readjusted PWM output signal by decreasing at least a portionof the level-shifted PWM output signal. The gate drive circuit mayinclude a MOSFET transistor having a source, a drain, and a gate,wherein the MOSFET transistor is electrically connected to the firstcharge pump and the second charge pump, wherein the level-shifted PWMoutput signal establishes an ON condition and the readjusted PWM outputsignal establishes an OFF condition of the MOSFET. The aspect of gatedrive circuit presented can be used in a variety of power convertertopologies including but not limited to DC/DC, DC/AC and AC/DCconverters. The functional characteristics of the circuit are validunder fixed or variable duty cycle conditions. For example, the providedcircuit can be used in isolated Zero Voltage Switching (ZVS) PhaseShifted Full Bridge type DC/DC converters where the operating duty cycleof the switching devices is always set to 50%. It is equally applicableto isolated and non-isolated DC/DC converters such as Flyback, buck andboost converters where the switch duty cycle can be different from 50%.It is also applicable in DC/AC inverters or AC/DC rectifiers where theduty cycle varies as a function of the waveshape of the output or theinput voltage waveform. Additional embodiments and variations of thegate driver circuit are further described herein.

FIG. 1 is block diagram of a gate drive circuit in accordance with anexample of the present disclosure. More specifically, FIG. 1 depicts A)a schematic of a gate drive circuit and B) a waveform of the gate drivecircuit. The power device to be switched on and off is designated asQ_(DUT). V_(ON) represents the voltage required at the gate to turn thedevice on while −V_(OFF) is the turn-off voltage. v_(dr) represents theinput to the gate drive circuit and v_(gs) may be the output voltage ofthe gate drive circuit and the input voltage applied to the gate ofQ_(DUT) for the required turn-on and turn-off levels. In one aspect, thespecifications of Q_(DUT) may indicate the voltage level required forturn-on is V_(ON) and the voltage required for turn-off is −V_(OFF). Inone aspect, the gate drive circuits as described herein may use acombination of transformer windings with varying turns-ratios and chargepumps to synthesize the turn-on and turn-off voltage levels at the gateof the switching device.

Turning now to FIG. 2A, an example schematic illustration of atransformer based gate drive circuit with additional secondary windingsis depicted. FIG. 2B is an example illustration of various waveforms forthe transformer based gate drive circuit of FIG. 2A. The transformerbased gate drive circuit 200 can include a gate drive transformer 201comprising a primary winding 204 responsive to a pulse width modulated(PWM) signal 202 (which may be an input signal). In one aspect, the PWMinput signal is a bipolar square wave, such as, for example, a symmetricbipolar square wave voltage. The gate drive transformer 201 can includea first secondary winding 208 and a second secondary winding 206responsive to a PWM signal 202 (which has now become a PWM outputsignal) of the primary winding 204. The first secondary winding 208 caninclude a first turn ratio different than a second turn ratio of thesecond secondary winding 206. The first secondary winding 208 canproduce a first bias voltage (V_(ON)) and the second secondary winding206 can produce a second bias voltage (V_(OFF)). In one aspect, acapacitor 210 and a diode 230 can be electrically connected to the firstsecondary winding 208. The capacitor 210 and diode 230 can be in seriesand/or in parallel to the first secondary winding 208 (N_(s1)). In oneaspect, a capacitor 212 and a diode 232 can be electrically connected tothe second secondary winding 206 (N_(s2)).

In one aspect, the gate drive transformer 201 can include additionalwindings 214, 216. The first additional winding 216 can be electricallycoupled to the first secondary winding 208. The second additionalwinding 214 can be electrically coupled to the second secondary winding206. The first additional winding 216 can be responsive to the firstbias voltage (V_(ON)). The second additional winding 214 can beresponsive to the second bias voltage (V_(OFF)) or (−V_(OFF)). In oneaspect, the first bias voltage (V_(ON)) produced from the firstadditional winding 216 can be a turn ON voltage, such as, for example aturn ON voltage that is at least twenty (20) volts. In an additionalaspect, the second bias voltage (V_(OFF)) produced from the secondadditional winding 214 can be a turn OFF voltage, such as, for example aturn OFF voltage that is at negative five (−5) volts.

A first drive MOSFET transistor 220 can be electrically coupled to thefirst additional winding 216 and responsive to the first bias voltage(V_(ON)). A second drive MOSFET transistor 222 can be electricallycoupled to the second additional winding 214 and responsive to thesecond bias voltage (V_(OFF)). Also, the first additional winding 216may be electrically connected to both resistors R₁₃ and R₂₃. The secondadditional winding 214 may be electrically connected to both resistorsR₁₄ and R₂₄.

In one aspect, the transformer based gate drive circuit 200 can alsoinclude a switching device 224 (e.g., a MOSFET transistor 224 or a IGBT,hereinafter “MOSFET transistor 224” for illustrative convenience) thatcan be electrically coupled to first drive MOSFET transistor 220 and thesecond drive MOSFET transistor 222. In one aspect, the first biasvoltage (V_(ON)) from the first drive MOSFET transistor 220 can drivethe MOSFET transistor 224 to turn it ON. In one aspect, the second biasvoltage V_(OFF) from the second drive MOSFET transistor 222 can drivethe MOSFET transistor 224 to turn it OFF. Furthermore, the MOSFET 224can be a silicon carbide MOSFET. MOSFET transistor 224 may beelectrically connected to both resistors R_(g) and R_(gs).

The power device required to be switched ON and OFF is Q_(DUT). Per thespecifications of Q_(DUT), the voltage level required for turn-on isV_(ON) and the voltage required for turn-off is −V_(OFF) (see FIG. 1).More specifically, as depicted in FIGS. 2A and 2B, the input to the gatedrive circuit may be represented by the voltage source v. As shown inFIG. 2B, v_(dr) may be a symmetric bipolar voltage applied to theprimary winding N_(p), 204 of the isolation transformer, T₁. TransformerT₁ may consists of 5 windings (204, 206, 208, 214, and 216), 1 primarywinding with N_(p) 204 turns and 4 secondary windings with turns N_(s1),N_(s2), N_(s3) and N_(s4) (206, 208, 214, and 216) as shown in FIG. 2A.Diode D₁ 230, capacitor C₁ 210, and diode D₂ 232, and capacitor C₂ 212are charge pumps that rectify the secondary voltages v_(s1) and v_(s2)and generate the turn-on and turn-off bias voltages V_(ON) and V_(OFF)respectively. Q₁ 220 and Q₄ 222 are MOSFETs or functionally equivalentdevices that apply voltages V_(ON) and V_(OFF) to the gate terminal ofQ_(DUT) to respectively turn-on and turn-off the device.

Voltages v_(s1) and v_(s2) generated across the secondary windings ofthe transformer T₁ are rectified by diodes D₁ 230 and D₂ 232respectively, which are given by Equation 1:

$\begin{matrix}{v_{{s\; 1},2} = {\frac{N_{{s\; 1},2}}{N_{p}}v_{dr}}} & (1)\end{matrix}$

Charge pump capacitors C1 210 and C2 212 charge to the peaks of thesquare wave voltages v_(s1) and v_(s2), respectively, to generatevoltages V_(ON) and V_(OFF). V_(ON) is the voltage level required toturn Q_(DUT) on and V_(OFF) is that required to turn Q_(DUT) off.

The turn-on and turn-off voltages (with the various wave formsillustrating positive (+) and negative (−) voltages in FIG. 2B) areapplied across the gate and source terminals of Q_(DUT) via the MOSFETsQ₃ 220 and Q₄ 222. Voltages V₃ and v₄ are generated from v_(dr)according to Equation 2.

$\begin{matrix}{v_{3,4} = {\frac{N_{{s\; 3},4}}{N_{p}}v_{dr}}} & (2)\end{matrix}$

Secondary windings N_(s3) 216 and N_(s4) 214 are wound in opposingdirections to each other such that voltage v₃ is in phase with v_(dr)while v₄ is 180° out of phase with v_(dr) As a result, MOSFETs Q₃ 220and Q₄ 222, which have voltages v₃ and v₄ applied to their gates areswitched complementary to each other. MOSFET Q₃ 220 is turned on toapply V_(ON) to the gate of Q_(DUT) while MOSFET Q₄ 222 is turned on toapply −V_(OFF) to the gate of Q_(DUT). The voltage v_(gs), applied tothe gate of the Q_(DUT) 224 is in phase with v_(dr). When v_(dr) isasserted to turn on Q_(DUT) v_(gs) is equal to V_(ON) and when v_(dr) isde-asserted to turn off Q_(DUT) v_(gs) is equal to −V_(OFF). It is notedthat no specific order is required in the methods disclosed herein,though generally in some embodiments, method steps can be carried outsequentially.

FIG. 3A is an example schematic illustration of a transformer based gatedrive circuit 300 with charge pumps for generating asymmetric drivevoltages. FIG. 3B is an example illustration of various waveforms 350for the transformer based gate drive circuit of FIG. 3A.

In one aspect, transformer based gate drive circuit 300 can include atleast one gate drive transformer 301. The gate drive transformer 301 caninclude a primary winding 304 (N_(p)) responsive to a pulse widthmodulated (PWM) signal 302 (e.g., voltage source v_(dr)) to generate abipolar signal having a positive bias voltage (e.g., first bias voltage(V_(ON))) and a negative bias voltage (e.g., second bias voltage(V_(OFF))). The PWM signal 302 can be a symmetric bipolar square wavevoltage. The positive bias voltage (e.g., first bias voltage V_(ON)))and the negative bias voltage (e.g., second bias voltage V_(OFF)) canhave a voltage range from positive thirteen (13) volts to negativethirteen (−13) volts.

The gate drive transformer 301 can also include a secondary winding 316N_(s) responsive to the bipolar signal to generate a PWM output signal.The PWM output signal can include the positive bias voltage and thenegative bias voltage.

In one aspect, the transformer based gate drive circuit 300 can includea first charge pump electrically connected to the secondary winding 316and responsive to the PWM output signal to generate a level shifted PWMoutput signal (e.g., an increased PWM output signal). The first chargepump can include capacitor (C₁) 306 and diode (D₁) (D₁) 310. Thetransformer based gate drive circuit 300 can also include a secondcharge pump electrically connected to the secondary winding 316 togenerate a readjusted PWM output signal by decreasing at least a portionof the increased PWM output signal. The second charge pump can includecapacitor C₂ 308 and diode D_(z) (D_(z)) 312, which can be a zenerdiode. The diode D. 312 may be electrically connected to resistor R_(z).The level shifted PWM output signal and the readjusted PWM output signalcan be bipolar square wave voltages. The level shifted PWM output signalcan have a voltage range from positive twenty six (26) volts to zero (0)volts. The readjusted PWM output signal can have a voltage range frompositive twenty (20) volts to negative six (−6) volts.

The transformer based gate drive circuit 300 can also include firstbipolar junction transistor (BJT) Q₃ 326 electrically connected to thefirst charge pump (collectively capacitor C₁ 306 and diode D₁ 310). Thetransformer based gate drive circuit 300 can also include a second BJTQ₄ 328 that can be electrically connected to the second charge pump(collectively diode D_(z) 314, capacitor C₂ 308, zener diode D_(z) 312).In one aspect, the zener diode D_(Z) 312 can control an amount of thelevel shifted PWM output signal to be decreased in order to generate thereadjusted PWM output signal.

The first BJT Q₃ 326 and the second BJT Q 328 can also be electricallyconnected to a gate switching device 324 (e.g., a gate MOSFET transistoror a IGBT, hereinafter “gate MOSFET transistor 324” for illustrative anddescriptive convenience). The first BJT Q₃ 326 and the second BJT Q₄ 328can each include a collector and a base. The collector of the first BJTQ₃ can be connected to a base of the first BJT using a first resistor R₃324 connected to the first charge pump (collectively capacitor C₁ 306and diode D₁ 310). A collector of the second BJT Q₄ 328 can be connectedto a base of the second BJT Q₄ 328 using a second resistor (R₄)(R₄) 320connected to the second charge pump (collectively diode D₂ 314,capacitor C₂ 308, zener diode D_(z) 312).

The gate MOSFET transistor 334 can have a source, a drain, and a gate.In one aspect, the first BJT Q₃ 326 can drive the level shifted PWMoutput signal into a gate of the MOSFET Q_(DUT) 334. The second BJT Q₄328 can also be electrically connected to the MOSFET Q_(DUT) 334. Thesecond BJT Q₄ 328 can drive the readjusted PWM output signal into thegate of the MOSFET Q_(DUT) 334. The MOSFET transistor Q_(DUT) 334 canhave a source, a drain, and a gate. The MOSFET transistor Q_(DUT) 334may also be connected to resistor R, 330 and resistor R_(gs) 332.

The level shifted (e.g., increased) PWM output signal driven from thefirst BJT Q₃ 326 to the gate MOSFET Q_(DUT) 334 can establish an ONcondition. The readjusted PWM output signal driven from the second BJTQ₄ 328 to the MOSFET Q_(DUT) 334 can establish an OFF condition of theMOSFET Q_(DUT) 334. The MOSFET Q_(DUT) 334 can be a silicon carbideMOSFET.

More specifically, the PWM input signal 302 to the gate drive circuit300 can be represented by the voltage source v_(dr). As shown in FIG.3B, voltage source v_(dr) may be a symmetric bipolar voltage applied tothe primary winding N_(p) of the isolation transformer, T₂. TransformerT₂ consists of 2 windings (304, 316), 1 primary winding 304 with N_(p)turns and one secondary winding 316 with N_(s) turns as shown in FIG.3A. A charge pump formed by diode D₁ 310 and capacitor C₁ 306 rectifiesthe secondary voltage and generates the turn-on voltage V_(ON) (with thevarious wave forms illustrating positive (+) and negative (−) voltagesin FIG. 3B). The turn-off voltage V_(OFF) is generated by the chargepump diode D₂ 314, capacitor C₂ 308, zener diode D_(z) 312. Thesecondary voltage V_(s) on the negative half cycle may reduced by thezener voltage V_(z) to generate the turn-off voltage (V_(OFF)) acrosscapacitor C₂. BJT Q₃ 226, diode D₃ 327, resistor R₃ 324 and BJT Q₄ 328,diode D₄ 322, and resistor R₄ form a complementary source follower todrive the required current into the gate of Q_(DUT). Neglecting diodeforward voltage drops, the voltage v, applied to the gate of Q_(DUT)(e.g., MOSFET 334) has the required turn-on and turn-off levels.

FIG. 4A is an additional example schematic illustration of a transformerbased gate drive circuit 400 with charge pumps for generating asymmetricdrive voltages. FIG. 4B is an example illustration of various waveforms450 for the transformer based gate drive circuit of FIG. 4A.

The transformer based gate drive circuit 400 can include gate drivetransformer 401 that can include a primary winding (N_(p)) 406responsive to a pulse width module (PWM) signal 402 (e.g., voltagesource v_(dr) to generate a bipolar signal having a positive biasvoltage and a negative bias voltage. The positive bias voltage can beasymmetric to the negative bias voltage. The positive bias voltage andthe negative bias voltage can have a voltage range from positivethirteen (13) volts to negative thirteen (−13) volts. The transformerbased gate drive circuit 400 can comprise a single secondary windingN_(s) 408 responsive to the bipolar signal to generate a PWM outputsignal. The PWM signal (e.g., PWM output signal) can include thepositive bias voltage and the negative bias voltage

In one aspect, the transformer based gate drive circuit 400 can includea first charge pump electrically connected to the secondary winding 408and responsive to the PWM output signal to generate a level shifted PWMoutput signal (e.g., an increased PWM output signal). The first chargepump can include capacitor C, 404 and diode D₁ 410. In one aspect, thecapacitor C₁ 404 and diode D, 410 can be in series and/or in parallel tothe secondary winding 408.

The transformer based gate drive circuit 400 can include a first bipolarjunction transistor (BJT) 418 (e.g. Q₃) and a second BJT 420 (e.g., Q₄)electrically connected to the first charge pump (collectively thecapacitor C, 404 and diode D, 410) to drive the level shifted PWM outputsignal. The first BJT Q₃ 418 and the second BJT Q₄ 420 can each includea collector and a base. The collector of the first BJT Q₃ 418 can beconnected to a base of the first BJT 418 using a first resistor R₃ 416that can be connected to the first charge pump (collectively capacitorC₁ 404 and diode D, 410). A collector of the second BJT Q₄ 420 can beconnected to a base of the second BJT Q₄ 420 using a second resistor R₄414 that is connected to the first charge pump (collectively capacitorC₁ (C₁) 404 and diode D₁ (D₁) 410). A diode can be substituted for BJTQ₃ 418 and resistor R₃ 416 by connecting the anode to the first chargepump (collectively capacitor C₁ 404 and diode D₁ 410) and the cathode tothe emitter of BJT Q₄ 420. This can result in reducing the physical sizeof the circuit. The first BJT Q₃ 418 and the second BJT Q₄ 420 can alsobe electrically connected to a second charge pump (collectivelycapacitor C_(zn) 424 and diode D_(zn) 422) in the transformer based gatedrive circuit 400. Resistor R_(g) 426 and resistor R_(gs) 428 may alsobe electrically connected to MOSFET 430 and capacitor C_(zn) 424 anddiode D_(zn) 422. That is, MOSFET Q_(DUT) 430 may be gate switchingdevice (e.g., a gate MOSFET transistor or a IGBT, hereinafter “MOSFET430” for illustrative and descriptive convenience).

The second charge pump (collectively capacitor C_(zn) 424 and diodeD_(zn) 422) can be electrically connected to first BJT 418 and thesecond BJT 420 to generate a readjusted PWM output signal by decreasingat least a portion of the level shifted PWM output signal. The secondcharge pump can include capacitor C_(zn) 424 and diode D_(zn) 422, whichcan be a zener diode.

In one aspect, the level shifted PWM output signal can have a voltagerange from positive twenty six (26) volts to zero (0) volts. Thereadjusted PWM output signal can have a voltage range from positivetwenty (20) volts to negative six (−6) volts.

The transformer based gate drive circuit 400 can include a gate MOSFETtransistor Q_(DUT) 430 having a source, a drain, and a gate. The gateMOSFET transistor Q_(DUT) 430 can be electrically connected to thesecond charge pump (collectively capacitor C_(zn) 424 and diode D_(zn)422). The readjusted PWM output signal at the gate MOSFET transistorQ_(DUT) 430 can establish an ON condition of the gate MOSFET transistorQ_(DUT) 430 and/or the readjusted PWM output signal at the gate MOSFETtransistor Q_(DUT) 430 can establish an OFF condition of the gate MOSFETtransistor Q_(DUT) 430.

The input to the gate drive circuit is represented by the voltage sourcev_(dr). As shown in FIG. 4B, v_(dr) is a symmetric bipolar voltageapplied to the primary winding N_(p) 406, of the isolation transformer,T₃. Transformer T₃ consists of 2 windings (406, 408), 1 primary winding(406) with N_(p) turns and one secondary windings 408 with N_(s) turnsas shown in FIG. 4B. A charge pump formed by diode D₁ 410 and capacitorC₁ 404 rectifies the secondary voltage and generates a level shiftedsquare wave voltage v₁. The turns ratio of the transformer T₃ is set toachieve V_(ON)+V_(OFF) VON+VOFF as the peak of voltage v₁. BJT 418 Q₃,resistor R₃ 416 and BJT 420 Q₄, resistor R₄ 414 represent acomplementary source follower to source and sink the required current inand out of the gate terminal of Q_(DUT) for turn-on and turn-offrespectively. Capacitor C_(zn) 424, zener diode D, 422 forms anothercharge pump. The zener voltage of D_(zn) is chosen to subtract V_(OFF)from the peak of v₁ V1 and apply it to the gate of Q_(DUT) to turn offthe device. Neglecting diode forward voltage drops, the voltage v_(gs)applied to the gate of Q_(DUT) has the required turn-on and turn-offlevels (with the various wave forms illustrating positive (+) andnegative (−) voltages in FIG. 4B).

It is to be understood that the embodiments of the invention disclosedare not limited to the particular structures, process steps, ormaterials disclosed herein, but are extended to equivalents thereof aswould be recognized by those ordinarily skilled in the relevant arts. Itshould also be understood that terminology employed herein is used forthe purpose of describing particular embodiments only and is notintended to be limiting.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, appearancesof the phrases “in one embodiment” or “in an embodiment” in variousplaces throughout this specification are not necessarily all referringto the same embodiment.

Various embodiments and example of the present invention may be referredto herein along with alternatives for the various components thereof. Itis understood that such embodiments, examples, and alternatives are notto be construed as de facto equivalents of one another, but are to beconsidered as separate and autonomous representations of the presentinvention.

Furthermore, the described features, structures, or characteristics maybe combined in any suitable manner in one or more embodiments. In thedescription, numerous specific details are provided, such as examples oflengths, widths, shapes, etc., to provide a thorough understanding ofembodiments of the invention. One skilled in the relevant art willrecognize, however, that the invention can be practiced without one ormore of the specific details, or with other methods, components,materials, etc. In other instances, well-known structures, materials, oroperations are not shown or described in detail to avoid obscuringaspects of the invention.

While the foregoing examples are illustrative of the principles of thepresent invention in one or more particular applications, it will beapparent to those of ordinary skill in the art that numerousmodifications in form, usage and details of implementation can be madewithout the exercise of inventive faculty, and without departing fromthe principles and concepts of the invention. Accordingly, it is notintended that the invention be limited, except as by the claims setforth below.

What is claimed is:
 1. A gate drive circuit for generating asymmetricdrive voltages, the gate drive circuit comprising; a gate drivetransformer comprising: a primary winding responsive to a pulse widthmodulated (PWM) input signal to generate a bipolar signal having apositive bias voltage and a negative bias voltage, the positive biasvoltage being asymmetric to the negative bias voltage; and a secondarywinding responsive to the bipolar signal to generate a PWM outputsignal; a first charge pump electrically connected to the secondarywinding responsive to the PWM output signal to generate a level-shiftedPWM output signal; a first transistor and a second transistorelectrically connected to the first charge pump to drive thelevel-shifted PWM output signal; a second charge pump electricallyconnected to the first transistor and the second transistor to generatea readjusted PWM output signal by decreasing at least a portion of thelevel-shifted PWM output signal; and a gate switching device having asource, a drain, and a gate, wherein the switching device iselectrically connected to the second charge pump, wherein the readjustedPWM output signal establishes at least one of an ON condition and an OFFcondition of the gate switching device.
 2. The gate drive circuit ofclaim 1, wherein the first transistor is a first Bipolar JunctionTransistor (BJTs) and the second transistor is a second BJT.
 3. The gatedrive circuit of claim 2, wherein the first Bipolar Junction Transistoris a diode.
 4. The gate drive circuit of claim 2, wherein a collector ofthe first BJT is connected to a base of the first BJT using a firstresistor connected to the first charge pump and a collector of thesecond BJT is connected to a base of the second BJT using a secondresistor connected to the first charge pump, and wherein the gateswitching device is at least one of a metal oxide semiconductor fieldeffect transistor (MOSFET) or an insulated gate bipolar transistor(IGBTs).
 5. The gate drive circuit of claim 1, wherein the positive biasvoltage and the negative bias voltage have a voltage range from positivethirteen (13) volts to negative thirteen (−13) volts, the PWM outputsignal includes the positive bias voltage and the negative bias voltage,the level shifted PWM output signal has a voltage range from positivetwenty six (26) volts to zero (0) volts, and the readjusted PWM outputsignal has a voltage range from positive twenty (20) volts to negativesix (−6) volts.